The present invention relates in general to on-line measurement of characteristics of webs of material as they are being manufactured and, more particularly, to a method and apparatus for on-line measurement of fiber orientation and anisotropy of non-woven webs of material as they are being manufactured. While the present invention is generally applicable to measurement of a variety of non-woven webs of material, it will be described herein with reference to the manufacture of paper webs for which it is particularly applicable and initially being applied.
Quality requirements for finished paper products continue to increase as the equipment handling such products becomes more and more technologically advanced. For example, modern printing presses and copy machines have significantly increased operating speeds. At the same time, persons using this equipment demand that they operate virtually fault free with no paper breaks, jams, misfeedings or the like. In addition to the higher speed of operation, the possibility for faults is further increased due to the intensive and fast heating and drying of paper for instance in modern copier machines and laser printers.
One characteristic of a paper web which has a large influence on the handling properties of paper produced from the web is the orientation of fibers of the web. A strong orientation in the machine direction dominating over the cross machine direction makes the web stronger in the longitudinal direction but weaker across the web or in the lateral direction. A fiber orientation that has a certain non-zero dominating angle relative to the machine direction commonly occurs due to flow patterns at the wet end of the paper making machine. Such non-zero fiber orientation often causes paper to have diagonal curl, twist, and the like. Of more concern, fiber orientation differences between the top surface and the bottom surface of a web will cause a strong skewing force that will make paper sheets exhibit curl whenever humidity changes. Curl, twist and other deformations due to improper fiber orientation result in problems when handling the paper such that the paper is unacceptable.
Fiber orientation has been measured in the prior art. Fiber orientation measurements are commonly done on finished paper products and thus can only be used to adjust paper making machines prospectively. Such measurements result in the production of substantial amounts of paper webs which may be unacceptable and do not permit control of the paper making machine during production to control fiber orientation. While some fiber orientation measurement techniques have been developed into on-line sensors for measuring one or both sides of a paper web, see for example U.S. Pat. No. 5,394,247 and published European Patent Application 0612977A2, applicant is unaware of any commercially successful on-line sensors which are in operation at this time.
Paper making machines and systems for controlling paper making machines have undergone significant improvements in an attempt to better monitor and control various characteristics of paper webs to thereby produce more and more consistent and higher and higher quality finished paper products. These modern machines provide the ability to dynamically control the quality of the paper web being produced including fiber orientation of the web. Unfortunately, up to this time, on-line measurement of fiber orientation and its control has been lacking.
Thus, there is a need for an economical, accurate on-line measurement for fiber orientation in machines producing non-woven webs of material, such as paper making machines. Preferably, such on-line measurement would measure both fiber orientation of the web and also fiber anisotropy. In addition, the on-line measurement would permit measuring fiber orientation both for the top of the web and for the bottom of the web.